The following abbreviations that may be found in the specification and/or the drawing figures are defined as follows:                3GPP third generation partnership project        CP cyclic prefix        D2D device to device (sometimes termed machine to machine M2M or peer-to-peer P2P)        DwPTS downlink pilot timeslot        DL downlink (eNB towards UE)        eNB EUTRAN Node B (evolved Node B)        GP guard period        LTE/LTE-A long term evolution/long term evolution-advanced        PDCCH physical downlink control channel        PUCCH physical uplink control channel        RAT radio access technology        RRC radio resource control        TDD time division duplex        UE user equipment        UL uplink (UE towards eNB)        UpPTS uplink pilot timeslot        
Research is ongoing into integrating new network topologies into cellular networks. For example, current discussions in LTE/LTE-A of 3GPP concern deploying a heterogeneous network of macros, micros, picos, femtos and relays in the same spectrum. One manner of doing this is two or more mobile devices and machines forming a local communication network under supervision of a cellular network. Such a supervised local network is one implementation of a D2D network, which might include the locally-linked devices performing certain tasks (e.g., spectrum sensing) in co-operative way. Generally the D2D devices need not have the same capabilities; for example an advanced device may act as a gateway for one or more low-capability devices or machines to access a broader network (e.g., cellular or Internet). A common theme in D2D is that the local D2D links represent a secondary usage of the cellular network's radio resources.
FIG. 1 illustrates an exemplary and non-limiting example of such a heterogeneous network. There is a cellular base station/eNB 101 and two or more UEs 102, 103 forming a D2D network amongst themselves. The D2D devices 102, 103 may each operate in the cellular communication mode with the base station 101 as well as in a local D2D mode with one another. In the cellular mode, links 102c and 103c carry data as well as control signaling. For purposes herein the cellular links 102c, 103c carry control information and so the D2D devices 102, 103 need not have an active data link with the cellular network. In the environment of FIG. 1, the UEs 102, 103 are in direct communication with one another over a primary D2D link 105p, which the cellular network 101 has allocated via the cellular links 102c, 103c for their D2D communications. By example, to save signaling overhead each such allocation may be sent in a single message (e.g., a PDCCH) addressed to a temporary identifier (e.g., a cell radio network temporary identity C-RNTI) assigned by the cellular network 101 to the D2D cluster, which each device 102, 103 of the cluster monitors.
One problem which may arise is that the D2D primary link 105d, which is allocated by the cellular network 101 for D2D communications, is insufficient for all the D2D data needs. By example this may result from high cellular demand for which the cellular network 101 may be obligated to give scheduling priority over D2D traffic, or simply from a high volume of D2D traffic. Some prior art approaches term the excess D2D traffic offload traffic.
Increasingly, mobile terminals such as those operating in the positions of the UEs 102, 103 of FIG. 1 are multi-radio devices with capability to operate on multiple different RATs (e.g., LTE, high-speed data packet access HSDPA, global system for mobile communication GSM, wireless local area networks WLAN also termed WiFi). It may be efficient where able to exploit these other RAT capabilities when able to handle the offload traffic.
One approach is to utilize cellular-network servers to assist offloading to WiFi, see FAST TRACK TO OFFLOAD CELLULAR DATA TO WIFI NETWORKS (http://www.notava.com/notava/uploads/Brochures/uAxesBrochure_v06.pdf, last visited Nov. 17, 2010). This may not always be the most flexible way of handling the local communication, for if devices are already in direct communication on a cellular-allocated primary D2D band 105p and want to W establish a new service between them which requires more resources, they can generally do the offloading procedure independently or with little assistance from the network.
Another offloading approach, detailed at co-owned U.S. patent application Ser. No. 12/782,877 filed on May 19, 2010, is for a network entity termed a D2D registration server function (DRSF) to assist with required RRC functionality for setting up and maintaining D2D communications between devices in 3GPP networks over both licensed bands and un-licensed bands. In that co-owned patent application the DRSF requests the D2D devices to make inter-RAT measurements on the unlicensed band for cellular band traffic offloading to the unlicensed band as decided by the network.
Such an unlicensed band for the offload traffic is shown at FIG. 1 as a secondary D2D link 105s. By example and unlike the primary D2D link 105p, the secondary D2D link 105s is not a cellular resource which is allocated by the network 101 directly but instead is compiled from ‘holes’ in the cellular licensed band which are found by spectrum sensing and opportunistically exploited for D2D use. The spectrum sensing assures that the D2D communications do not interfere with the primary users, those operating within the cellular system on radio resources specifically allocated by the network 101 which controls those resources.
Various specifics as to what is sensed are known in the art; a signal level exceeding a threshold, energy detection, (cyclostationary) feature detection, correlation, etc. How to arrange this sensing is more constrained, given the limited power reserves and processing capacity of the D2D devices 102, 103 and the desire to avoid high signaling overhead in sharing any sensing results among the D2D devices which utilize them. The exemplary embodiments detailed herein detail an efficient arrangement for organizing spectrum sensing, which may be used to find the D2D secondary link 105s of FIG. 1 onto which offload traffic might be ported. Such embodiments may also be practiced where there is no network-allocated primary D2D link 105p, in which the relevant traffic would not be characterized as offload.